Image generation elements in printers can be as varied as inkjet nozzles, dot matrix elements, or LED arrays in laser printers. Such elements operate in response to driving signals which are generated in a printing pipeline which converts image data to signals which cause the image generation elements to reproduce the image on a medium with greater or lesser degrees of fidelity. (The medium may be a medium on which the image is directly printed (e.g. paper) or it may be an intermediate medium such as the photosensitive drum of a laser printer from which the final printing is carried out.)
A “printing pipeline” as defined herein is a collection of components or a succession of processes which operate on image data (such as, for example PostScript or Portable Document Format (PDF) files [both PostScript and PDF are Trade Marks of Adobe Systems], contone image data or halftone image data) to generate signals which are effective to directly or indirectly drive a number of image generation elements. Typically, the printing pipeline might include the components responsible for colour mapping an image file, linearising the image data, creating a halftone image from the linearised contone image, applying a print mask to the halftone image and outputting drive signals for driving the image generation elements (e.g. inkjet nozzles in an inkjet printhead) to reproduce the intended image. The printing pipeline is not limited to this specific collection of steps which is given as an example of the typical processes involved.
Conventional inkjet printers employ one or more printheads mounted on a carriage which is repeatedly scanned across a scan axis as the print medium is advanced stepwise past the scan axis. The printheads lay down swaths of ink during each scan, between advances of the print medium.
In a conventional high speed printer, each page of a print job is colour mapped (converted from a computer output format such as Postscript™ or Portable Document Format™ to a contone map of pixels) by a dedicated raster image processor (RIP).
The rasterised pages are buffered and then processed by an application specific integrated circuit (ASIC) which converts the bitmap into a halftone image composed of halftone data. The most basic colour printer will use combinations of cyan, magenta and yellow (CMY) to make the various colours, or for increased quality true black ink may be also available (CMYK). For high quality images, two additional inks, light cyan and light magenta are also employed to provide increased fidelity particularly with lighter tones such as skin tones (CcMmYK printing).
Each ink is conventionally printed by a separate printhead. The printheads are controlled by a printhead controller which analyses the halftone image data and specifies the nozzle firing sequence to cause the printheads to lay down ink at the correct point on the page, so that the printed image will be a more or less faithful reproduction of the original input image.
A continuing goal of inkjet printing technology is to improve printing speeds, which are limited by a number of factors including the time taken to scan across the page, which may be increased in multiples where multiple pass print modes are employed (in which each area of the printed image is covered by a multiple number of swaths to improve print quality).
One way of reducing print times is to employ a page wide array (PWA) of printheads. In PWA printers, an array of printheads extending across the width of the page is maintained in a static position during printing and the medium advances under the PWA, eliminating scanning times. As the number of printheads increases however, the processing requirements of the printer similarly increase.
In particular a bottleneck is likely to arise between the buffer in which the rasterised pages are stored and the ASIC which creates the halftone image, since the ASIC must operate in real time to supply the printhead controllers with the halftone image which they operate on to create the print mask for their own printheads.
A further problem which arises in PWA printers is related to the fact that arrays of image generation elements are unlikely in practice to operate identically across the array. In the case of inkjet printers, the nozzles are generally grouped in printheads which are each defined on a die or wafer. The dies are subject to manufacturing variations, and this can lead to variations in the volume of droplet ejected (which in turn affects image quality significantly).
Such problems are not limited to inkjet printers. A laser printer having an array of LEDs illuminating a photosensitive drum (such that the characteristics of the drum change affecting the adhesion of toner to the drum) will also suffer an analogous problem if the performance of the LEDs varies under the application of the same driving signals.